This routine designs a band-pass diplexer filter, that has a constant
input impedance of Zo at all frequencies, both in the pass-band and in
the stop-band. S11 is always good. S22 is poor, except near the centre.
------------------L1-----------------C1---------------------
           |             
           |----------|
           |          |
1->       C2         L2                                   <- 2
           |          |
           |----------
           |            
           R            
           |            
------------------------------------------------------------

